Electromagnetic couplings and clutches are well known. In one application, electromagnetic clutches are employed with a belt driven pulley in motor vehicles to drive accessory equipment when the clutch is engaged. A typical application is an air conditioner compressor that is periodically operated in response to air conditioning controls.
Generally, a single belt driven by a vehicle engine is trained over several pulleys to drive a number of accessories, some of which may employ a clutch for occasional operation. If one of the accessories employing a clutch should become jammed or otherwise malfunction so that its shaft does not rotate, the belt and the other accessories driven by the same belt may be adversely affected and ultimately fail. To avoid those failures, it is desirable to detect clutch slippage.
A particularly useful electromagnetic clutch employs a "soft start" that gradually brings an accessory up to full rotational speed and avoids an abrupt engagement. One clutch of that type is described in commonly assigned U.S. Pat. No. 4,749,073 to Olsen, the disclosure of which is incorporated herein by reference.
An apparatus for sensing slippage in an electromagnetic clutch is disclosed in U.S. Pat. No. 4,627,525 to Geldec. The Geldec apparatus constantly generates a pulse train or alternating current signal, the frequency of which is monitored. Changes in the monitored pulse frequency indicate speed changes that are related to clutch slippage. The Geldec apparatus requires a frequency counter and frequency discrimination circuitry, making it expensive and complex. Accordingly, it is desirable to provide an apparatus and method for detecting slippage in an electromagnetic clutch that is simple and inexpensive.
Another apparatus for sensing slippage in an electromagnetic clutch is disclosed in U.S. Pat. No. 4,572,343 to Boffelli. That clutch is intended for use with power take-offs on farm tractors. The clutch includes a rotor and an armature containing radial notches and slots, respectively. When the engaged clutch slips, the notches and slots rotate relative to each other, producing variable magnetic reluctance paths. The flux produced by a winding that engages the clutch varies in response to the variable reluctance paths. The varying flux is detected to indicate clutch slippage.